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Carrier rooftop limit switch

Got a Carrier rooftop, model 48TM008, that is going off on the limit switch. Another guy changed it in December, and today was the first time, that I know of, that it went off again. I put my digital thermometer in the box so it was as close to the disc as possible. The unit ran for the hour and a half I was watching it. Temp on the digital got up to about a max of 172F. It's rated for 130, with a 40 degree diff. Whats the normal operating temp for the limit? Seems to run fine during the day, but when it does go off, it is either late at night/early morning, because we get the calls first thing in the morning. Could we possibly be looking at a bad heat exchanger, leaking and heating up the disc? Any thoughts? Thanks.

Got a Carrier rooftop, model 48TM008, that is going off on the limit switch. Another guy changed it in December, and today was the first time, that I know of, that it went off again. I put my digital thermometer in the box so it was as close to the disc as possible. The unit ran for the hour and a half I was watching it. Temp on the digital got up to about a max of 172F. It's rated for 130, with a 40 degree diff. Whats the normal operating temp for the limit? Seems to run fine during the day, but when it does go off, it is either late at night/early morning, because we get the calls first thing in the morning. Could we possibly be looking at a bad heat exchanger, leaking and heating up the disc? Any thoughts? Thanks.

Got a Carrier rooftop, model 48TM008, that is going off on the limit switch. Another guy changed it in December, and today was the first time, that I know of, that it went off again. I put my digital thermometer in the box so it was as close to the disc as possible. The unit ran for the hour and a half I was watching it. Temp on the digital got up to about a max of 172F. It's rated for 130, with a 40 degree diff. Whats the normal operating temp for the limit? Seems to run fine during the day, but when it does go off, it is either late at night/early morning, because we get the calls first thing in the morning. Could we possibly be looking at a bad heat exchanger, leaking and heating up the disc? Any thoughts? Thanks.

Which limit are you referring to? You have one that sits on a plate above the burner area and one that sits on the base plate between the blower and the heat exchanger.

A good rule for all carrier rtu's, is to check the heat exchangers every visit. They seem to have gotten better, but a 48tm008 pull that cover off the heat exchanger and check it out. Also I know the configuration is crap, but pull the burner rack and clean it up.

close to same problem i had. rtu had bad filter in ceiling. not enough return to have enough air to push the heat off the heat exchangers (limit open). I always ask what temp to they leave it overnight. some people expect the it to be 62'f and hit 75'f in 10min. just an idea.

What's your actual temp rise?
What's your actual gas pressure at the gas valve?
Are your returns clean and not restricting airflow?
Are your supplys too close to your retuns, short cycling heated air?
Is this an 8 or 10ft ceiling, stratisfied heat returning to unit?
Is the heat ex cracked or corroded at the seams?

With a 172°F measured temp at the limit, assuming your return temp is approx 75 to 80°F, you're right around a 100° rise. That a bit high. Check name plate for rated temp rise